In the art of making hot pressed or sintered ceramics, graphite has been frequently used to form the elements of the hot pressing or sintering fixture, the fixture being used to contain the ceramic materials for densification. Graphite is selected for such use because it is relatively strong at high temperature conditions (i.e., 1000.degree.-2000.degree. C.), it has a low modulus of elasticity at high temperatures, and, most importantly, it is excellent for coupling with an induction magnetic field to indirectly heat the ceramic material contained within the graphite elements (see U.S. Pat. Nos. 3,632,708 and 4,071,372). Unfortunately, the graphite frequently reacts with the ceramic material at the high temperature and/or pressure conditions of hot pressing or sintering, forming an unwanted material on the fixture which must be removed for subsequent processing. In sintering, this has been indirectly alleviated by packing of the ceramic material to be sintered in a loose, discardable powder (see U.S. Pat. No. 4,179,486) which may add to the cost of complexity of processing.
In the art of nitriding silicon particles to form silicon nitride by gas phase reaction in a heated furnace, graphite has been used only as a structural wall or felt material located remotely from the silicon to be nitrided, thus avoiding the unwanted reaction by intimate contact with the silicon powder (see U.S. Pat. No. 3,244,480).
It would be desirable in the processing of silicon (to a fully dense silicon nitride object) to combine the arts of cold compaction, nitriding, and hot pressing/or sintering in a continuous procedure. This would obviate frequent handling of the materials to be worked and frequent rearrangement of the processed materials, all of which is costly and can promote flaws if not carried out correctly.